The clamp part of the thinggy is clearly 'clamped' by a nut and bolt that goes through both arms and presumably through the part that it's clamped to. From the relative size of the rule (bloody metric......!) I estimate that the clamp fits over a SMLE nose cap and the retaining bolt clamps it through the nose cap screw. What the swinging wing thing is for is anyones guess. But that's started the ball rolling

Second thinggy. Wot about 3" mortar striker? Is the rounded striker part spring loaded, operated by a cord through the broken end of the plunger at the right hand end?